Biodegradable dispersants for cement compositions and methods of cementing in subterranean formations

ABSTRACT

Biodegradable dispersants for cement slurries, well cement compositions comprising such biodegradable dispersants, and methods of using such well cement compositions are provided. The biodegradable dispersants of the present invention comprise low molecular weight starches that comprise anionic groups.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to subterranean cementing operations, and more particularly, to improved cement dispersants for cement compositions and methods of cementing in subterranean formations.

[0003] 2. Description of the Prior Art

[0004] Hydraulic cement compositions are commonly utilized in subterranean applications including but not limited to well completion and remedial operations. For example, in well applications, hydraulic cement compositions are used in primary cementing operations whereby strings of pipe such as casing and liners are cemented in well bores. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and/or positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions are also used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks in holes in pipe strings, and the like.

[0005] Dispersants are often used in subterranean well cement compositions to facilitate mixing the cement composition. Such dispersants are extensively used, inter alia, to reduce the apparent viscosities of the cement compositions in which they are utilized to allow the cement composition to be pumped with less friction pressure and less horsepower. In addition, the lower viscosity often allows the cement composition to be pumped in turbulent flow. Turbulent flow characteristics are desirable, for instance, when pumping cement compositions into subterranean wells to more efficiently remove drilling fluid from surfaces in the well bore as the drilling fluid is displaced by the cement composition being pumped. The inclusion of dispersants in cement compositions is also desirable in that the presence of the dispersants may facilitate the mixing of the cement compositions and reduce the water required. This may be desirable because cement compositions having reduced water content are often characterized by improved compressive strength development.

[0006] A number of dispersing agents have been utilized heretofore in cement compositions, particularly in cement compositions used for primary and remedial cementing in subterranean wells. One of the most common cement slurry dispersants is a condensate product of sulfonated naphthalene and formaldehyde. Such dispersants are problematic, however, because they are not substantially biodegradable, and hence, do not meet the regulatory requirements in some countries for use as dispersants.

[0007] Another conventional cement composition dispersant is the condensation product of formaldehyde, acetone, and an alkali metal sulfite. One formulation of this conventional dispersant is commercially available under the trade designation “CFR-3” from Halliburton Energy Services, Inc., of Duncan, Okla. While this and other similar dispersants may function well as dispersants in cement compositions, they are often environmentally unacceptable for use in wells subject to more stringent environmental regulations. Their unacceptability in these environments stems from, inter alia, their inability to undergo complete biodegradation in the environment, which may result in undesirable environmental effects if either accidentally or intentionally released into the environment.

SUMMARY OF THE INVENTION

[0008] The present invention provides biodegradable cement slurry dispersants, cement compositions comprising such dispersants, and methods of cementing in a subterranean formation.

[0009] In certain embodiments, the biodegradable dispersants of the present invention comprise a low molecular weight starch that comprises anionic groups.

[0010] In one embodiment, the methods of cementing in a subterranean formation of the present invention comprise providing a cement composition comprising a hydraulic cement, water, and a biodegradable dispersant of the present invention; placing the cement composition in a subterranean formation; and allowing the cement composition to set therein.

[0011] The objects, features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the preferred embodiments that follows.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0012] The biodegradable dispersants of the present invention comprise a starch-based material. Starch is a polymer of glucose in which monosaccharide units are linked by 1,4′-α-glycoside bonds. Starches suitable for use in accordance with the present invention may be obtained from renewable glucose sources such as potatoes, corn, maize, tapioca, other cereal grains, and the like. Combinations of starches from combinations of sources are also suitable. The starch-based dispersants of the present invention are advantageous, because they are, inter alia, biodegradable and do not cause undesirable effects on the environment.

[0013] The starches of the present invention preferably should be characterized by a low molecular weight, in the range of about 500 to about 10,000, or more preferably about 1,000 to about 5,000. Examples of low molecular weight starches include dextrinized starches and starches degraded with acids, peroxides, or other oxidizing agents. For dextrinized starches, heat or acid treatment methods may reduce the molecular weight of the starch. These starches can be used with or without reacting them with suitable reagent groups to add anionic groups that may increase their dispersing ability as discussed below.

[0014] In the case of non-oxidized, low molecular weight starches such as acid, peroxide, or heat degraded starches, the starch can be further reacted with suitable reagents to introduce anionic groups. Similarly, to enhance the dispersing action of any suitable starch, the starch may be modified to incorporate anionic charges. Anionic groups that may be useful include but are not limited to: sulfite, sulfate, sulfonate, carboxylate, silicate, nitrate, and nitrite groups. These anionic groups can be added to the starch by reacting the starch with an agent capable of producing the anionic groups, including but not limited to hypochlorite, peracetic acid, hydrogen peroxide, sodium periodate, sodium persulfate, sodium percarbonate, propane sultone, butane sultone, chloroacetic acid, dinitrogen tetroxide, 3-chloropropyl-triethoxy silane, 3-glycidoxy-propyltrimethoxy-silane, or the like.

[0015] Additionally, when the starch contains substantially carbonyl groups, particularly aldehyde groups, it can be further reacted with a sulfite salt to provide a sulfite adduct of an oxidized starch; sulfite adducts of oxidized starches are especially suitable for use in accordance with the present invention. Another method of forming a sulfite adduct of an oxidized starch includes reacting an acetone formaldehyde condensate with starch under alkaline conditions, followed by addition of a sulfite salt as described in U.S. Pat. No. 5,247,086, incorporated herein by reference. Other suitable starches include propylene oxide modified starches, ethylene oxide modified starches, and lightly crosslinked starches.

[0016] In preferred embodiments of the present invention, the starch in the dispersant is oxidized. Oxidation of starches either with hypochlorite, hydrogen peroxide, sodium periodate, or the like is suitable as such oxidation is likely to produce starch molecules that have carbonyl groups, particularly aldehydes, and carboxylate groups. When the oxidized starch comprises carboxylate groups, the material itself may be used as dispersant or further reacted with a sulfite salt to produce dispersant containing both sulfite and carboxylate anionic groups.

[0017] The anionic groups discussed above are thought to provide, inter alia, dispersing -action in highly viscous slurries, e.g., slurries having a low water to cement ratio, or slurries containing a large fraction of solids, without substantially affecting the time in which the cement composition sets therein.

[0018] The solubility and storage characteristics of the dispersants of the present invention depend at least in part on the structural features of the starch or starches utilized. For example, one type of starch contains two components, namely a linear polymer molecule referred to as amylose, and a branched polymer molecule referred to as amylopectin. High amylose starches are typically not cold-water soluble. Heating such starch suspensions usually dissolves the starch. Upon cooling, the solutions become viscous gels. Additionally, these solutions may continue to viscosify with time due to the alignment of linear starch molecules in a process called retrogradation. Such high amylose starches, when modified to function as dispersants for cement slurries, may be more suitable for dry blending. On the other hand, high amylopectin-containing starches are significantly more cold-water soluble and the solutions are less likely to gel at room temperature. Such starches may be more amenable to a liquid form. Additionally, as will be recognized by those skilled in the art with the benefit of this disclosure, the water solubility of starches can further be increased by introducing low levels of cationic groups in the form of quaternary ammonium groups. Such cationizations are carried out by reacting starch with, e.g., glycidylpropyltrialkylammonium salts. This may decrease the tendency of a starch to gel while stored. Other modified starches useful in the present invention include propylene oxide modified starches, ethylene oxide modified starches, and cross-linked starches.

[0019] The biodegradable dispersants of the present invention may be utilized in any subterranean cementing applications. All cements suitable for use in subterranean cementing operations may be used in accordance with the present invention. In one embodiment, the improved cement compositions of the present invention comprise a hydraulic cement, sufficient water to form a pumpable slurry, and a biodegradable starch-based dispersant of the present invention in an amount effective to reduce the apparent viscosity of the cement composition prior to when it gels and sets. In certain embodiments, the biodegradable dispersant of the present invention is included in the cement composition in the amount of from about 0.01% to about 5.0%. In other embodiments, the dispersant may be added in a range of from about 0.2% to 3.0%. A variety of hydraulic cements can be utilized in accordance with the present invention including those comprised of calcium, aluminum, silicon, oxygen, and/or sulfur, which set and harden by reaction with water. Such hydraulic cements include, but are not limited to, Portland cements, pozzolana cements, gypsum cements, high alumina content cements, silica cements, and high alkalinity cements. Portland cements are generally preferred. In some embodiments, the Portland cements that are suited for use in conjunction with the present invention are classified as Class A, C, H, and G cements according to American Petroleum Institute, API Specification for Materials and Testing for Well Cements, API Specification 10, Fifth Ed., Jul. 1, 1990. Another useful cement for certain embodiments of the present invention include a cement that is commercial available under the tradename “THERMALOCK™” from Halliburton Energy Services, Inc., in Duncan, Okla., and described in U.S. Pat. No. 6,488,763, incorporated herein by reference. The dispersants of the present invention are also suitable for use with low-density cements. Such low-density cements may be foamed cements or may be cements comprising another means to reduce their density such as hollow microspheres, low-density elastic beads, or other density-reducing additives known in the art.

[0020] The dispersants of the present invention can be used in a liquid or solid form, depending on the application. For example, a dispersant of the present invention in a powder form can be blended with dry cement prior to mixing with water. Such dry blends are generally preferred when the wells to be cemented are on land. In offshore applications, it is preferred that the dispersants are in a solution or suspension form.

[0021] The water utilized in the cement compositions of this invention can be fresh water, salt water (e.g., water containing one or more salts dissolved therein), brine (e.g., saturated salt water produced from subterranean formations), or seawater. Generally, the water can be from any source provided that it does not contain an excess of compounds that adversely affect other components in the cement composition. The water preferably is present in an amount sufficient to form a pumpable slurry. More particularly, the water is present in the cement compositions in an amount in the range of from about 30% to about 110% by weight of hydraulic cement therein, more preferably in an amount of about 40%.

[0022] One example of a composition of the cement compositions of the present invention is comprised of a hydraulic cement; water present in an amount in the range of from about 30 to about 110% by weight of cement in the composition; and a starch-based biodegradable dispersant of the present invention, present in an amount in the range of from about 0.2 to about 3.0% by weight of cement in the composition.

[0023] As will be recognized by those skilled in the art, when the cement compositions of the present invention are utilized for primary or remedial subterranean well operation, such compositions can also include additional additives such as fluid loss additives, weighting materials, light weight material, set retarders, accelerators, and the like. The cement compositions also can include other additives such as accelerators or retarders, if desired. If an accelerant is used, it is preferably calcium chloride. Also, if used, in certain embodiments, such accelerants are present in an amount in the range from about 1.0% to about 4.0% by weight of the cement in the compositions. Fluid loss additives such as hydroxyethylcellulose, carboxymethylcellulose, carboxymethylhydroxyethylcellulose, hydroxypropylcellulose, hydroxypropylguar, guar, polyvinylalcohol, synthetic polyelectrolytes are also suitable.

[0024] To facilitate a better understanding of the present invention, the following examples of preferred embodiments are given. In no way should such examples be read to limit the scope of the invention.

EXAMPLES

[0025] A number of test cement compositions were prepared using a Standard Grade Class A hydraulic cement mixed with a sufficient amount of fresh water to form a pumpable slurry. One of the test compositions contained a conventional dispersant, specifically CFR-3 dispersant, and other samples contained examples of dispersants of the present invention. Another control sample was tested wherein no dispersant was added to the cement composition. In addition, to illustrate the effects of oxidizing the starch, another control sample was tested wherein the starch in the starch-based dispersant was not oxidized prior to combining it with the cement.

[0026] Tables 1, 2 and 3 list the characteristics of the illustrative dispersants of the present invention that were tested. Table 4 indicates how much and which type of the dispersant of the present invention were added to a given cement composition. Tables 5A and 5B list the rheological properties of the cement compositions comprising the dispersants of the present invention. The Comparative Tables below illustrate the properties of the control samples discussed above. TABLE 1 Examples of Dispersants of the Present Invention. Reaction Reaction Weight % Weight % Weight % Temperature Time Sample Starch Starch Bisulfite Sulfite (° C.) (hours) A Dialdehyde Starch 1.7 1.9 — 56 16 B Dialdehyde Starch 3.8 — 5.2 63 8.5 C Dialdehyde Starch 5.3 — 3.6 72 5

[0027] Dispersant No. A was made as follows. A suspension of a dialdehyde starch (1.7% by weight) purchased from Sigma-Aldrich Chemical Company in water was mixed with sodium bisulfite (1.9% by weight) and reacted at 56° C. for 16 hours. The dialdehyde starch is an oxidized starch. The concentration of solids in the final mixture was 8.39%; the solution density was 1.05 g/cc. The reaction mixture was a colorless solution. 1.0% by weight of this dispersant was added to a standard cement mixture to form Cement Composition No. 2 shown in Table 4.

[0028] Dispersant No. B was made as follows. A suspension of a dialdehyde starch (3.8% by weight) from Sigma-Aldrich Chemical Company was mixed with 5.2% by weight sodium sulfite and 225.4 grams of water at 63° C. for 8.5 hours to obtain a dark brown colored suspension. The resultant suspension had a density of 1.2 g/cc and a solids concentration of 10%. 1.0% by weight of this dispersant was added to a standard cement mixture to form Cement Composition No. 3 shown in Table 4.

[0029] Dispersant No. C was made as follows. A suspension of a dialdehyde starch (5.3% by weight) from Sigma-Aldrich Chemical Company was mixed with 3.6% by weight sodium sulfite and 225.4 grams of water. The components were reacted at 72° C. for 5 hours to obtain a dark brown colored suspension. The resultant suspension had a density of 1.2 g/cc and a solids concentration of 10%. 0.2% by weight of this dispersant was added to a standard cement mixture to form Cement Composition No. 7 shown in Table 4. TABLE 2 Examples of Dispersants of the Present Invention. Weight % Weight % Weight % Reaction Reaction Sample Starch Starch Hypochlorite Sulfite Temp. Time D High Amylose 8.2 0.8 4.7 53 3 Starch¹ E High Amylose 8.1 0.8 4.6 53 24 Starch¹

[0030] Dispersant No. D was made as follows. A suspension of a high amylose starch was obtained from National Starch. 8.2% by weight of the starch was mixed with 0.8% of hypochlorite to oxidize the starch. The oxidized starch was reacted with 4.7% by weight of sodium sulfite at 53° C. for 3 hours to produce the dispersant. The concentration of solids in the dispersant was 11.5% and the density was 1.07 g/cc. The pH was 9.3. 0.2% by weight of this dispersant was added to a standard cement mixture to form Cement Composition No. 9 shown in Table 4.

[0031] Dispersant No. E was made as follows. A suspension of a high amylose starch obtained from National Starch and Chemical Company containing 8.1% by weight of the starch was mixed with 0.8% of hypochlorite to oxidize the starch. The oxidized starch was reacted with 4.6% by weight of sodium sulfite at 53° C. for 24 hours to produce the dispersant. The concentration of solids in the dispersant was 12.7% and the density was 1.06 g/cc. The pH was 7.1. 0.2% by weight of this dispersant was added to a standard cement mixture to form Cement Composition No. 10 shown in Table 4. TABLE 3 Examples of Dispersants of the Present Invention. Weight % Weight % Weight % Reaction Reaction Sample Starch Starch Periodate Sulfite Temp. Time F High Amylose Starch² 7.3 0.8 0.5 72 24 G High Amylose Starch² 7.3 0.8 0.5 72 24

[0032] Dispersant No. F was made as follows. A suspension of a high amylose starch was obtained from National Starch and Chemical Company containing 7.3% by weight of the starch. This was mixed with 0.8% of periodate to oxidize the starch and allowed to react for about 16 hours. The oxidized starch was reacted with 0.5% by weight of sodium sulfite at 72° C. for 24 hours to produce the dispersant. The concentration of solids in the dispersant was 7.8% and the density was 1.05 g/cc. 0.2% by weight of this dispersant was added to a standard cement mixture to form Cement Composition No. 11 shown in Table 4.

[0033] Dispersant No. G was made as follows. A suspension of a high amylose starch was obtained from National Starch and Chemical Company containing 7.3% by weight of the starch mixed with 0.8% of periodate to oxidize the starch and 0.5% by weight of sodium sulfite at 72° C. for 24 hours to produce the dispersant. The concentration of solids in the dispersant was 7.8% and the density was 1.05 g/cc. 0.2% by weight of this dispersant was added to a standard cement mixture to form Cement Composition No. 12 shown in Table 4.

[0034] To examine the effect of these examples of the dispersants of the present invention on the rheology of a typical cement slurry, each of these dispersants was added to a standard cement mixture. The cement mixture for each sample was a mix of a conventional hydraulic cement commonly used in subterranean well applications and a sufficient amount of water to form a slurry. A sufficient amount of each dispersant was added to the cement/water slurry to form a slurry having about 16.4 lbs/gallon density. Table 4 lists the particular dispersant and the amount of which was added to the cement slurry. The particular dispersants are listed above in Tables 1-3. TABLE 4 Cement Compositions Comprising Examples of Dispersants of the Present Invention. Weight % of Dispersant Cement Added to Composition Dispersant Cement 2 A 1.0% 3 B 1.0% 4 C 0.2% 5 D 0.2% 7 E 0.2% 8 F 1.0% 9 G 0.2% 10 H 0.2% 11 I 0.2% 12 J 0.2%

[0035] Tables 5A and 5B list the rheological properties of each of these cement compositions containing the illustrative dispersants of the present invention. The rheological properties of the test cement compositions were determined in accordance with the Recommended Practice for Testing Well Cements, API Recommended Practice 10B, 22nd Edition, dated December, 1997, of the American Petroleum Institute. TABLE 5A Rheological Properties of Cement Slurries Comprising Dispersants of the Present Invention Shown in Table 4. Cement Cement Cement Cement Cement Compo- Compo- Compo- Compo- Compo- Viscosity, sition sition sition sition sition (cp) No. 2 No. 3 No. 4 No. 5 No. 7 300 rpm 49 33 49.5 43 46 200 rpm 34 22 36 31 33.5 100 rpm 18 11 23 19 20  60 rpm 12 7 17 14 14  30 rpm 7 4 14 10 9.5  6 rpm 3 2 8 6.5 5.5  3 rpm 3 1.5 7 6 5 600 rpm 99 73 93 64 93

[0036] TABLE 5B Rheological Properties of Cement Slurries Comprising Dispersants of the Present Invention Shown in Table 4. Cement Cement Cement Cement Cement Compo- Compo- Compo- Compo- Compo- Viscosity, sition sition sition sition sition (cp) No. 8 No. 9 No. 10 No. 11 No. 12 300 rpm 39 41 43 43 74 200 rpm 26 28 30 30 55 100 rpm 13.5 15 18 16 36  60 rpm 8.5 11 13 11 28  30 rpm 5 6 9 7 23  6 rpm 1.5 3.5 6 4.5 19  3 rpm 1 3 5.5 4 16 600 rpm 85 82 84 not 125 determined

[0037] Another example was performed to illustrate, inter alia, that the sulfite adducts of an oxidized starch will disperse cement slurries. A 100 g sample of a 10% suspension in water of an oxidized starch, “D17F” obtained from Grain Processing Corp., of Muscatine, Iowa, was heated to 185° F. for 6 hours, at which time the suspension became stable to settling. The pH of the suspension was adjusted to 12.2, and 0.75 g of sodium sulfite was added. The mixture was heated to 185° F. for 4 hours. The resulting light yellow turbid mixture was tested in the cement slurry for its dispersing activity as follows. For comparison, a sample of the starch suspension after adjusting the pH but prior to the addition of sodium sulfite was also tested.

[0038] A cement slurry having a density of 16.4 lbs/gal containing 0.53% of the control oxidized starch by weight of the cement and another similar slurry containing the sulfite adduct of an oxidized starch were prepared and tested for rheology. The results are shown below in Table 6. TABLE 6 Rheological Properties of Cement Slurries Comprising Dispersants of the Present Invention. Viscosity Control Oxidized Starch Sulfite Reacted Oxidized Starch 300 rpm 90 79 200 rpm 70 58 100 rpm 45 33  6 rpm 22 10  3 rpm 16 11 600 rpm 150 150

[0039] From Table 6, it is evident that the sulfite addition product of an oxidized starch disperses cement slurries better than the control oxidized starch.

[0040] To determine how these illustrative dispersants of the present invention compared to conventional nonbiodegradable dispersants, a comparative test was run. To perform these tests, the same standard cement composition was made as described above (hydraulic cement and a sufficient amount of water to form a slurry) but a conventional widely accepted nonbiodegradable dispersant was added. This nonbiodegradable dispersant was CFR-3. 0.2% by weight of the dispersant was added to the cement composition. The resultant cement composition had a density of 16.4 lbs/gal. COMPARATIVE TABLE 1 Control Sample, Cement Composition Containing Conventional Nonbiodegradable Dispersant. Cement % by weight of Composition No. Dispersant Type dispersant 6 Conventional CFR-3 0.2%

[0041] The rheological properties of the cement composition containing the conventional nonbiodegradeable dispersant (Cement Composition No. 6) were determined as reported in Comparative Table 2. COMPARATIVE TABLE 2 Rheological Characteristics of Cement Composition 6 Containing Conventional CFR-3 Composition in Comparative Table 1. RPM Viscosity, cp 300 rpm 46 200 rpm 34 100 rpm 20  60 rpm 14.5  30 rpm 10  6 rpm 7  3 rpm 6.5 600 rpm 100

[0042] As can be seen from Tables 5A, 5B, and Comparative Table 2, the biodegradable dispersants of the present invention imparted properties to cement compositions which compare closely with the properties imparted to the same cement compositions comprising a highly-accepted conventional nonbiodegradable dispersant. In particular, Cement Composition Nos. 5, 7, 9, 10, and 11 particularly appear to impart comparable properties to the cement compositions as the conventional nonbiodegradable dispersant.

[0043] Another control sample was also tested to determine the effectiveness of the illustrative dispersants of the present invention. In this second control sample, no dispersant was added to the cement mixture. The cement mixture contained hydraulic cement and water in an amount sufficient to form a slurry, and had a density of 16.4 lbs/gal. Comparative Table 3 lists the characteristics of this cement mixture. COMPARATIVE TABLE 3 Control Sample, Cement Composition Having No Dispersant. Composition No. Dispersant Type % by weight of dispersant 1 None None

[0044] After forming this control sample with no dispersant, the rheological properties of the cement were determined. These are reported in Comparative Table 4. COMPARATIVE TABLE 4 Rheological Properties for Control Sample of Comparative Table 3. RPM Viscosity, cp 300 rpm 102 200 rpm 87 100 rpm 68  60 rpm 60  30 rpm 50  6 rpm 24  3 rpm 17 600 rpm 164

[0045] As can readily be seen from examining Tables 5A, 5B, and Comparative Table 4, the dispersants of the present invention proved to be effective in reducing the viscosity of the cement mixture to a desirable level.

[0046] Another control test was performed to determine the effect of oxidizing the starch prior to reacting it with a sulfite. The control starch in this test was used to prepare dispersants D and E in Table 2. Reacting the oxidized starch with a sulfite is thought to provide anionic sulfite groups to the starch. The anionic sulfite groups are thought to contribute to the dispersing action of the dispersant in highly viscous slurries, e.g., slurries having a low water to cement ratio, or slurries containing a large fraction of solids. COMPARATIVE TABLE 5 Cement Composition Wherein the Dispersant is a Starch That Was Not Oxidized or Reacted With a Sulfite. % by weight of Composition No. Dispersant Type dispersant 13 Starch Solution, National Starch 0.2% High Amylose Starch (Not Oxidized)

[0047] COMPARATIVE TABLE 6 Rheological Properties for Control Sample of Comparative Table 5. RPM Viscosity, cp 300 rpm 81 200 rpm 53 100 rpm 27  60 rpm 18  30 rpm 10  6 rpm 10  3 rpm 11 600 rpm 135

[0048] An examination of data for compositions 9 and 10 in Table 5B and Comparative Table 6 reveals that oxidizing the starch followed by reaction with a sulfite ion improves the dispersing characteristics of the starch-based dispersants of the present invention.

[0049] A high amylopectin starch sample derived from waxy maize containing siloxy anionic groups and cationic quaternary ammonium groups, available from National Starch and Chemical Company under the trade name “COBOND 2500” as a 15% starch solution in water was tested in the same cement slurry at a level of 1% by weight of cement. The rheology results are shown in Table 7. COMPARATIVE TABLE 7 Rheological Properties of Cement Slurry Comprising a Dispersant of the Present Invention. RPM Viscosity, cp 300 rpm 100 200 rpm 75 100 rpm 46  6 rpm 23  3 rpm 20 600 rpm 170

[0050] A comparative slurry without Cobond 2500 provided the following rheology in Comparative Table 7. COMPARATIVE TABLE 7 Control Slurry Having No Dispersant. RPM Viscosity, cp 300 rpm 115 200 rpm 102 100 rpm 79  6 rpm 25  3 rpm 18 600 rpm 213

[0051] The results from using siloxy-substituted starch show that siloxy anion substituted starches also disperse cement slurries.

[0052] Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those that are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims. 

What is claimed is:
 1. A method of cementing in a subterranean formation comprising the steps of: providing a cement composition comprising a cement, water, and a dispersant comprising a low molecular weight starch that comprises anionic groups; placing the cement composition in the subterranean formation; and allowing the cement composition to set therein.
 2. The method of claim 1 wherein the dispersant comprises a starch derived from potatoes, corn, maize, tapioca, or other cereal grains.
 3. The method of claim 1 wherein the starch has a molecular weight of about 1,000 to about 5,000.
 4. The method of claim 1 wherein the starch is a dextrinized starch or a starch degraded with an oxidizing agent or heat.
 5. The method of claim 1 wherein the starch is a dialdehyde starch, a high amylose starch, or a high amylopectin starch.
 6. The method of claim 1 wherein the starch comprises sulfite, sulfate, sulfonate, carboxylate, silicate, nitrate, or nitrite groups.
 7. The method of claim 1 wherein the starch is a sulfite adduct of an oxidized starch.
 8. The method of claim 1 wherein the dispersant is present in the cement composition in an amount sufficient to reduce the apparent viscosity of the cement composition prior to when the cement composition sets.
 9. The method of claim 8 wherein the dispersant is present in the cement composition in an amount of from about 0.01% to about 5%.
 10. The method of claim 1 wherein the cement is a hydraulic cement.
 11. The method of claim 10 wherein the hydraulic cement comprises calcium, aluminum, silicon, oxygen, or sulfur.
 12. The method of claim 1 wherein the hydraulic cement is a Class A, Class C, Class H, or a Class G cement.
 13. The method of claim 1 wherein the cement is a low-density cement.
 14. The method of claim 1 wherein the water is fresh water, salt water, or brine.
 15. The method of claim 1 wherein the water is present in an amount in the range of from about 30% to about 110% by weight of the cement in the cement composition.
 16. The method of claim 1 wherein the cement is a hydraulic cement, the water is present in an amount from about 30% to about 110% by weight of the cement in the cement composition, and a dispersant is present in an amount in the range of from about 0.2% to about 3.0% by weight of the cement in the cement composition.
 17. The method of claim 1 wherein the cement composition further comprises a fluid loss additive, weighting material, light weight material, set retarder, or an accelerator.
 18. A method of reducing the viscosity of a cement composition comprising the step of adding a dispersant to the cement composition wherein the dispersant comprises a low molecular weight starch having anionic groups.
 19. The method of claim 18 wherein the dispersant comprises a starch derived from potatoes, corn, maize, tapioca, or other cereal grains.
 20. The method of claim 18 wherein the starch has a molecular weight of about 1,000 to about 5,000.
 21. The method of claim 18 wherein the dispersant comprises a dextrinized starch or a starch degraded with an oxidizing agent or heat.
 22. The method of claim 18 wherein the starch is a dialdehyde starch, a high amylose starch, or a high amylopectin starch.
 23. The method of claim 18 wherein the starch comprises sulfite, sulfate, sulfonate, carboxylate, silicate, nitrate, or nitrite groups.
 24. The method of claim 18 wherein the starch is a sulfite adduct of an oxidized starch.
 25. The method of claim 18 wherein the dispersant is present in the cement composition in an amount of from about 0.01% to about 5%.
 26. The method of claim 18 wherein the dispersant is present in the cement composition in an amount sufficient to reduce the apparent viscosity of the cement composition prior to when the cement composition sets.
 27. A cement composition comprising a low molecular weight starch that comprises anionic groups.
 28. The composition of claim 27 wherein the starch is derived from potatoes, corn, maize, tapioca, or other cereal grains.
 29. The composition of claim 27 wherein the starch has a molecular weight of about 1,000 to about 5,000.
 30. The composition of claim 27 wherein the starch comprises a dextrinized starch or a starch degraded with an oxidizing agent or heat.
 31. The composition of claim 27 wherein the starch comprises sulfite, sulfate, sulfonate, carboxylate, silicate, nitrate, or nitrite groups.
 32. The composition of claim 27 wherein the starch is a sulfite adduct of an oxidized starch.
 33. The composition of claim 27 wherein the starch is a dialdehyde starch, a high amylose starch, or a high amylopectin starch.
 34. The composition of claim 27 wherein the starch is present in the cement composition in an amount sufficient to reduce the apparent viscosity of the cement composition prior to when the cement composition sets.
 35. The composition of claim 27 wherein the starch is present in the cement composition in an amount of from about 0.01% to about 5%.
 36. The composition of claim 27 wherein the cement composition comprises a hydraulic cement.
 37. The composition of claim 36 wherein the cement comprises calcium, aluminum, silicon, oxygen, or sulfur.
 38. The composition of claim 36 wherein the cement is a Class A, Class C, Class H, or a Class G cement.
 39. The composition of claim 36 wherein the cement is a low-density cement.
 40. The composition of claim 27 wherein the cement composition comprises water selected from the group consisting of fresh water, salt water, and brine.
 41. The composition of claim 40 wherein the water is present in an amount in the range of from about 30% to about 110% by weight of cement in the cement composition.
 42. The composition of claim 27 wherein the cement composition comprises a hydraulic cement, water present in an amount from about 30% to about 110% by weight of the cement in the cement composition, and wherein the starch is present in an amount in the range of from about 0.2% to about 3.0% by weight of the cement in the cement composition.
 43. The composition of claim 27 further comprising a fluid loss additive, weighting material, light weight material, set retarder, or an accelerator. 